SA212 troubleshouting

[jujub]

HI everyone

Was playing with my amp à low volume.... hten it sudenly fade the now there no more sound of it.

tube are lighting... tranny are not hot..

Hope it not the patridge

what can I do ?

[mhuss]

Did you check the HT fuse? That fuse only interrupts the high voltage, the tubes and indicator will still light if it is blown.
http://hiwatt.org/tech.html

On very rare occasions, this fuse will fail due to age & vibration, but it most often indicates a bad output tube (EL34). If the fuse is blown, replace it, but watch the EL34s when you turn it on again. One may be red-plating (the tube structure itself starts glowing a dull red, different from the bright orange heaters). This condition somestimes causes extra hum from the speakers, but otherwise there's no audible indication.

If a tube did red-plate for a while, it may be permanently damaged, i.e., the HT fuse will blow again quickly.

In any case, I suggest you remove the chassis and inspect the output tube sockets for any sign of overheating or charring. There is a small resistor connected to the socket that typically burns/turns black when an output fails catastrophically.

[jujub]

thanks !

Ht fuse is ok, therefor tube are red plating ( mullards XF2 ) ...

my be a problem with the bias...bias cap is original...

[mhuss]

Are you sure the fuse is OK? Sometimes (particularly with vibration failure) the fuse looks perfectly fine, but measures open.

[jujub]

Fuse have been tested ok

Could it be a préam tube? PI?

[jujub]

Amp is working again !

There was just a broken wire ... the HT1 connexion to the output tranny. HERE




I wonder how this happen

Thanks.

[mhuss]

I'm glad you found it, and that it was a simple fix.